This invention relates in general to a process for depositing thin films in the fabrication of semiconductor devices and more specifically to a process for depositing high purity dielectric and passivation thin films on a semiconductor substrate.
The processing of semiconductor devices often requires the deposition of dielectric and passivation films. The dielectric and pasivation films are used to electrically isolate two or more conductive layers from each other and from a conductive substrate. Integrated circuits often use insulated gate field effect transistors (IGFETs) having a conducting gate electrically isolated from a semiconductor substrate by a thin dielectric material. For instance, in CMOS semiconductor devices this conducting gate may be polycrystalline silicon (polysilicon) overlying a silicon oxide dielectric. Dielectrics are also used in capacitors, such as for DRAM memories, and to separate first and second polysilicon layers overlying a semiconductor substrate. As integrated circuits have been scaled down to accommodate ever increasing densities of semiconductor devices a corresponding decrease in dielectric film thickness is necessary. The reduction of film thickness into the range of 100 angstroms or less has reduced the acceptable defect tolerance level of these films. The VLSI environment in which the dielectric films must function requires that these films be of high dielectric strength, have a high breakdown voltage, be radiation resistant, and provide a diffusion barrier to contaminants such as sodium and dopants used to adjust the electrical conductivity of adjacent conductive layers.
In the case of VLSI MOS device fabrication, the dielectric and insulating films are commonly composed of silicon dioxide or silicon nitride, dielectric films can also be composed of a composite structure consisting of oxide-nitride-oxide (ONO). Silicon nitride, in addition to having suitable dielectric properties is an excellent barrier to sodium diffusion. The diffusion barrier property of silicon nitride has increased the application of this material for use as a dielectric structure in semiconductor devices both singularly and in combination with silicon dioxide.
Silicon nitride for application as a dielectric or a passivation material can be deposited by reacting silane with ammonia at a temperature of about 700 to 900 degrees centigrade in an atmospheric pressure chemical vapor deposition (CVD) reactor. For example, silicon nitride is formed according to the following reaction, EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2 (1)
A more uniform nitride deposition on a given substrate may be obtained by the reaction of dichlorosilane with ammonia at a reduced pressure (0.25 to 2.0 torr) and a temperature range of 700 to 800 degrees centigrade. For example, silicon nitride is formed at low pressure according to the reaction, EQU 3SiCl.sub.2 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl+6H.sub.2 (2)
The formation of a high quality film by either CVD or low pressure CVD (LPCVD) requires that the constituents be incorporated in the film in approximately stoichiometric proportions. However, excess ammonia is normally used in order to avoid the formation of a silicon-rich nitride film. The incorporation of excess silicon in the nitride film has the deleterious effect of reducing electrical resistance of the silicon nitride thereby diminishing its dielectric properties. The introduction of excess ammonia in the reactor is effective in preventing the incorporation of excess silicon in the silicon nitride film, however, the use of excess ammonia results in the formation of a silicon nitride film which can have a hydrogen content of up to about 8 gram-atomic %. The presence of hydrogen is undesirable because hydrogen enhances the diffusion of contaminants into and through the silicon nitride dielectric.
A similar contamination problem occurs when a silicon dioxide film is deposited in and LPCVD reactor. For example, a silicon dioxide film is formed by reacting silane with oxygen at about 400 to 500 degrees centigrade according to the reaction, EQU SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2 (3)
Alternatively, silicon dioxide may be formed in an LPCVD reactor by the reaction of dichlorosilane with nitrous oxide at about 900 degrees centigrade according to the reaction, EQU SiCl.sub.2 H.sub.2 +2N.sub.2 O.fwdarw.SiO.sub.2 +2HCl (4)
The formation of a silicon dioxide film by LPCVD deposition yields a film containing silicon hydroxide (SiOH) in the range of about 1 to 4 weight percent. As in the case of silicon nitride, the presence of hydrogen in the silicon dioxide results in an enhancement of the films ability to transport contaminants.
From the foregoing it is apparent that all of the methods described above yield a hydrogenated film. The presence of hydrogen seriously compromises the sodium barrier characteristics of a nitride film and leads to the inclusion of contaminants in a silicon dioxide film. The deleterious effects of contamination induced defects in dielectric and passivation films used in semiconductor devices is well known in the semiconductor process sciences. Defects form interface charge traps which can shift the threshold voltage of an MOS transistor and reduce the charge storage capability of a DRAM capacitor. VLSI devices can incur serious reliability problems due to the presence of interface states originating from contamination in a dielectric film. Accordingly, a need existed for a method of producing contamination free dielectric and passivation films for use in semiconductor device fabrication.